A resonator fiber optic gyroscope (RFOG) is a rotation sensing device that comprises a fiber resonant ring cavity. When the gyro has nonzero rotation rate around an axis normal to the plane of the ring cavity (also called resonator), the effective round trip path lengths for lightwaves propagating in the two opposite directions are different. This so-called Sagnac effect causes the ring cavity to exhibit different resonance frequencies for the two opposite directions which are often referred to as clockwise (CW) and counter-clockwise (CCW) for convenience. The resonance frequency difference is proportional to the gyro rotation rate.
To measure the resonance frequency difference of the ring cavity in CW and CCW directions, monochromatic lightwaves are coupled into the resonator in both directions and their frequencies are separately tuned/locked to the peaks (or dips, depends on how light is coupled out of the resonator) of the resonance characteristics. The term “resonance characteristic” or simply “resonance” of a ring cavity refers to the curve of resonator output power as a function of input light frequency. This curve typically has a periodic pattern of peaks (or dips) whose corresponding frequencies are the resonance frequencies. The frequency difference between two adjacent peaks (or dips) of the same spatial-mode and polarization-mode of the resonator, just differing by one wavelength of optical pathlength within the resonator, is the resonator free spectral range (FSR). It is determined by the round trip propagation time of the resonator.
For precise resonance frequency measurement, the resonator cavity should support only a single spatial mode and a single polarization mode so that signal errors generated from unwanted spatial and polarization modes can be negligible. In prior art inventions, intra-resonator polarizers with high polarization extinction ratio (PER) have been suggested to suppress the unwanted polarization modes. However, the resonator optical fibers are often assumed to support pure single spatial mode and few discussions of the impact of high order spatial mode were presented. Recently there are increasing interests in using bandgap hollow core fiber in fiber optic gyroscopes to take its advantages (compared to traditional solid core fibers) of low sensitivity to environmental changes, negligible nonlinear refraction index, and low phase fluctuations. During the research of hollow core fiber gyroscopes, it was found that the current commercially available low loss hollow core fiber may weakly support high order spatial modes, sometimes by design, and sometimes due to imperfections in either design or manufacturing process. Finding the mitigation methods of the impact of high order spatial mode on gyro performance becomes increasingly important.
In cases where resonator loop-fibers weakly support high order spatial modes, i.e. modes other than the fundamental one, the impact on gyro bias stability may be significant. Due to modal dispersion, high order modes travel in different optical paths and have different round-trip phase delays with respect to the fundamental mode. When the resonator output light is detected, light from the higher order mode may interfere with the fundamental mode, producing resonance asymmetry that varies with environmental changes. Although this interference may be suppressed to some degree by the spatial averaging of two spatially-orthogonal modes, the suppression may be compromised by imperfect spatial averaging; e.g. when the detected light is traveling through a finite aperture or the detector has spatial inhomogeneities. Although possibly weak in intensity, the high order modes can be a severe limiting factor of bias stability of a high performance RFOG.